Acute immobilization stress results in a decrease in adrenaline levels in brain stem nuclei. These changes occur in brain stem regions probably related to the changes observed in spontaneously hypertensive rats. Adrenaline changes were more pronounced and widespread than those on noradrenaline, indicating that those two catecholamines may have different roles regarding the cardiovascular regulatory mechanisms and the response to stress. Serotonin and histamine appear to be concentrated in areas of the brain stem where changes in catecholamines occur during hypertension.